This invention relates to seals and gaskets, and more particularly to a spiral wound gasket. The spiral wound gasket of the present invention includes improvements that reduce the problem of inner buckling of the gasket when it is compressed during installation.
Spiral wound gaskets are well known in the industry. Typically, a spiral wound gasket includes a thin metal strip between which strips of filler material are interspersed as the strip is wound upon itself forming a winding about a central axis. A number of different metals can be used for the thin metallic strip. Each material provides its own particular characteristics to meet desired specifications. Likewise, various filler materials can be used. Typically, the filler material employed is a soft strip of material that is deformed when the spiral wound gasket is compressed between pipe flanges thereby providing desired sealing characteristics.
Spiral wound gaskets are normally disposed between opposed flanges of mating pipe ends. The pipe flanges are clamped together by means of circumferentially spaced bolts or any other suitable fastening arrangement. In many situations, outer guide rings are used in conjunction with and become part of a spiral wound gasket assembly. The outer guide rings are mounted around the outside circumference of a spiral wound gasket. The outer guide rings are usually formed from carbon steel and serve a number of different functions in the gasket assembly. A primary function of the outer guide ring is to act as a compression limiter, so the gasket assembly is not compressed beyond design limits. Additionally, the outer guide ring provides increased radial strength to the gasket assembly. Moreover, the outer guide ring facilitates installation by providing a positive centering of the gasket on the pipe flange.
By design, a spiral wound gasket can be compressed from its original manufactured thickness of approximately 0.175" down to the outer guide ring thickness of 0.125'. As the spiral wound gasket is compressed two things are occurring. First of all, depending upon the compressibility of the filler material, the filler itself is being compressed such that there is an overall reduction in the volume of the gasket element. Once the filler is compressed to its "full density" there can be no further reduction in the gasket volume. Compression beyond this point merely displaces the fixed volume of the gasket. Three predominate filler materials used today are mica-graphite, flexible graphite and PTFE. While both the mica-graphite and flexible graphite are compressible and will allow some degree of volume reduction within the gasket as it is being compressed, sintered PTFE is essentially uncompressible and the compression of a spiral wound gasket with this filler results only in a displacement of the original volume. However, due to the lack of control that exists with conventional gasket winding equipment, much of the initial compressibility that exists with the graphite filler materials is reduced as the gasket is being produced, rendering it essentially incompressible even before the gasket is installed in a flange. To enhance the mechanical reliability and sealing performance of gaskets today, gaskets are installed using much higher bolt loads than were typically used in the past. These higher bolt loads are overcoming the resistance of the fully compressed filler/gasket element and forcing volume displacement as the gasket is deflected down to the thickness of the outer retaining ring. The implosion of the gasket at the inside diameter, otherwise referred to as inner buckling, is the result.
There are substantial problems or concerns as a result of inner buckling. First of all, when the gasket buckles during installation there is a corresponding loss of bolt load because of the stress relief that has occurred. Secondly, a protrusion of the gasket into the pipe bore not only creates turbulent flow, but it is also likely to break the gasket and cause "unwinding" into the flow stream, ultimately creating a total loss of seal. An object called a "pipe pig" is often shot through pipe runs to clear any scale or clogs along the inside of the pipe. Along with normal flow through the pipe, use of a pipe pig can also break the gasket and cause it to unwind and fail if it has buckled into the inside diameter.
A separate inner retaining ring was once considered to be a solution to the inward buckling of spiral wound gaskets. Inner rings have become a requirement in national standards (ASME B16.20) on many sizes and filler styles of spiral wound gaskets to aid in resisting the distortion of the gasket in the radially inward direction. For instance, all spiral wound gaskets having PTFE as a filler material are required to have an inner ring. Now it is recognized, however, that this alone will not prevent inward buckling. While these inner rings impede the displacement or flow of the gasket into the inside diameter of the pipe, they are physically unable to completely prevent this inward flow because of their narrow cross section. The inside diameter of the gasket remains as the weakest plane. Unfortunately, these inner rings add considerably to the cost of the spiral wound gasket. These increased costs result from the cost of the metal itself (typically a stainless steel or exotic alloy), machining costs, labor costs to install it and finally the cost of inventorying a separate line item. Also, their fit within the spiral wound gasket inside diameter is often variable. Often times they fall out during handling or shipping, creating a sense of unreliability.
Another phenomena that occurs when there are extreme radial forces that are developed during compression is "dishing" of the outer guide ring. The normally flat outer guide ring becomes dished, or forced into a convex or concave shape, as high radial forces are developed. As the ring becomes dished, still higher bolt loads must be exerted upon it to render it flat again so that it performs as a true 0. 125' compression stop.